The invention relates to a method for operating a direct oxidation fuel cell in which at least one fluid fuel is transported from a fuel reservoir via a fluid distribution structure to a membrane electrode assembly, the transport of the fuel being effected passively, i.e. without convection. Furthermore, the invention relates to a corresponding direct oxidation fuel cell.
The energy requirement of electrical small consumers (PDA, mobile phone etc.) is constantly increasing. However whilst microprocessors have become approx. thirty times faster in the past ten years, the energy density of batteries has merely doubled. The fuel cell is regarded in this context as a possible replacement or as a supplement for conventional batteries and accumulators. In particular direct oxidation fuel cells (e.g. direct methanol fuel cells) are seen as particularly promising mobile energy sources since the liquid fuel cell can be handled comparatively easily and generally has an energy density higher by a multiple than batteries or accumulators.
One of the essential challenges for operation of a direct oxidation fuel cell resides in the production of carbon dioxide bubbles during the oxidation of the carbon-containing, liquid fuel at the anode-side. There may be mentioned as example the oxidation of methanol with the help of water: CH3OH+H2O→CO2+6 H++6 e. These bubbles can become fixed within the gas diffusion layer or in the fluid distribution structure and thus reduce the active cell surface and the performance of the fuel cell.
In a standard mode of operation, a pump which provides the direct oxidation fuel cell with liquid fuel is operated continuously. The gas bubbles are dissolved in liquid with the generally superstoichiometric volume flow or are rinsed out of the fuel cell with the liquid and are separated in a subsequent step from the liquid fuel. This requires continuous operation of the pump which is accompanied by fairly large power requirements of the pump and consequently reduces the efficiency of the system of the entire fuel cell system.
Starting from the state of the art of continuously operated systems, it was the object of the present invention to improve the operation of direct oxidation fuel cells such that a higher system efficiency of the fuel cell can be achieved. In addition, a system was intended to be provided by means of which the so-called cross-over of fuel, i.e. the penetration of unconsumed fuel through the membrane, is prevented.
This object is achieved by the generic method having the characterising features of claim 1 and the generic direct oxidation fuel cell having the characterising features of claim 15. The further dependent claims reveal advantageous developments.